The first Transistor was invented in 1948 by W.H. Brattain and J. Bardeen; transistor has gained good use, they are used in almost most electronic applications. Though a transistor is only slightly more than 70 years old, and it has replaced old vacuum tubes in almost all applications. In this post, we will cover What is BJT (Bipolar Junction Transistor) Step by Step, we will also learn working of BJT as an Amplifier and Switch and their Applications.

Hey friends, in this article we are going to learn about What is BJT or Bipolar Junction Transistor, Working as an Amplifier and Switch and their Applications, So far we have covered diodes and their all types.

As we know “a diode is a simple P-N Junction device and it has two doped region (N-region) and (P-region) when a second P-N junction is formed and connected in the back to back fashion then the resulting device is known as BJT (Bipolar Junction Transistor).

What is BJT (Bipolar Junction Transistor)?

BJT Definition:

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A BJT or Bipolar Junction Transistor is a three-terminal device. It has two P-N junctions and it’s mostly used for amplification purpose. It amplifies the weak signals. 

It is the current controlling device. (By varying the Base current the voltage across collector terminal will be varied hence it is known as a current controlling device).

  1. The word transistor is the combination of two words one “Transfer” and other “Resistor”. This describes the operation of a BJT. i.e. the transfer of an input signal from low resistance to output high resistance.
  2. The transistor is a semiconductor device hence it is made up of Silicon (Si) or Germanium (Ge).

Construction of BJT (Bipolar Junction Transistor):

A transistor is consists of two PN junction which is formed by *sandwiching either N-type or P-type semiconductor between a pair of opposite types of layers, which forms three regions called emitter, base, and collector.

Symbol of BJT

(i) symbol of BJT, (ii) Back to Back connection of Diode forming BJT

Types of BJT (Bipolar Junction Transistor)

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According to the construction of BJT (Bipolar Junction Transistor), there are two types of BJTs:

1: NPN Transistor:

An N-P-N transistor is composed of two N-type layers separated by a thin layer of the p-type region as shown in Fig given below.

NPN Bipolar Junction Transistor

2: PNP Transistor:

A P-N-P transistor is composed of two P-type layers separated by a thin layer of the N-type region as shown in Fig given below.

PNP Bipolar Junction Transistor

Terminals of BJT (Bipolar Junction Transistor)

A BJT transistor (PNP or NPN) has three doped semiconductor sections. The section on one side is called an emitter and the section on the opposite side is called the collector.

The middle section is called the base and this forms two junctions between the emitter and collector.

(i) Emitter:

The emitter terminal supplies charge carriers (holes or electrons). The emitter is always made forward biased w.r.t. base so that it can supply a large number of majority charge carriers.

  • Emitter Terminal according to region wise is much wider than the base but less wide than the collector.
  • Emitter Terminal according to doping; is heavily doped so that it can inject a large number of charge carriers.

In PNP transistor: The emitter (p-type) of PNP transistor is forward biased and supplies hole charges (which are majority carriers in case of P-region) to its junction with the base.

In NPN transistor: The emitter (n-type) of NPN transistor has a forward bias and supplies free electrons to its junction with the base.

(ii) Collector:

This terminal collects the charges hence it is called the collector. The collector is always made reverse biased. Its function is to remove charges from its junction with the base.

  • Collector Terminal according to region wise is much wider than both terminals.
  • Collector Terminal according to doping; is moderately doped.

In PNP transistor: The collector (p-type) of PNP transistor has a reverse bias and receives hole that flows in the output circuit.

In NPN transistor: the collector (n-type) of NPN transistor has reverse bias and receives electrons in the output circuit.
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PNP and NPN Bipolar Junction Transistor

(i) PNP Bipolar Junction Transistor, (ii) NPN Bipolar Junction Transistor

(iii) Base:

The middle section which forms two PN-junctions between the emitter and collector terminal is called the base. The base-emitter junction is forward biased, allowing low input resistance for the emitter circuit.

The base-collector junction is reverse biased and provides high resistance at the output in the collector circuit.

  • Base Terminal according to region wise is much thinner than the emitter.
  • Base Terminal according to doping; is lightly doped and very thin and it passes most of the emitter charge carriers to the collector.

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Operating Region of BJT (Bipolar Junction Transistor):

The BJT (bipolar junction transistors) have the ability to operate in three different regions:

Active Region   –   in the active region the BJT transistor operates as an amplifier and its input junction is forward bias and its output junction is reverse bias.

Saturation  –   in the Saturation region BJT transistor is “Fully-ON” operating as an ON switch and its both junction are in Forward Bias.

Cut-off  –   in the Cut-off region the BJT transistor is “Fully-OFF” operating as an OFF switch and its both junctions are in reverse bias.


operation region of bjt

Working of BJT (Bipolar Junction Transistor) as an Amplifier:

Active Region

“To operate Bipolar Junction Transistor as an Amplifier we have to make input Base-Emitter junction of BJT in forward bias and output junction Base-Collector junction to reverse bias”.

How is the Amplification happening in the BJT Transistor?

In case of NPN transistor:

Given figure shows the NPN transistor with forward bias to Base-Emitter junction and reverse bias to Base-Collector junction.

Due to the forward bias at Base-Emitter junction electrons in the N-type emitter starts to flow towards the base terminal and hence this constitutes the emitter current IE. As the base is P-type and the electrons, they tend to combine with holes. As the base terminal is less doped and very thin, therefore, only a very few electrons (less than 4%) combine with holes to constitute base current IB.

The remainder (more than 96%) electrons cross over into the collector-region to constitute collector current IC. In this way, it is observed that almost the entire emitter current flows in the collector circuit. And hence the emitter current is the sum of collector and base currents i.e. IE = IB+ IC
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NPN Bipolar Junction Transistor operations


In case of PNP transistor:

This is very same like NPN working but the majority carriers will be different in this operation. Given figure shows the basic connection of a PNP transistor. Due to the forward bias; the holes in the p-type emitter starts to flow towards the base of N-type. This constitutes the emitter current IE.

As these holes cross into the N-region base, they tend to combine with the electrons in the N-region.

hence the base is less doped and its also very thin, therefore, only a few holes (less than 4%) combine with the electrons. The remaining electrons (more than 96%) cross into the collector region to constitute collector current IC.

PNP Bipolar Junction Transistor operation

In this way, it is observed that almost the entire emitter current flows in the collector circuit.

What happened in reality when we made Forward bias Base-Emitter junction and Reverse bias Base-Collector Junction?

As we learned about an Active region in which the input circuit Base-Emitter is forward bias hence it has very low resistance because of forward bias whereas output circuit Base-Collector was in reverse bias and hence it has high resistance. As we have seen above, the input emitter current is almost equal to the collector current. (IE=IB+IC) because emitter current almost entirely flows in the collector circuit.

Therefore, a transistor transfers the input signal current from a low-resistance circuit to the output high-resistance circuit.

And this is the main factor responsible for the amplification capability of the transistor. I hope it’s more clear now!

BJT (Bipolar Junction Transistor) as an Amplifier:

A BJT transistor amplifier increases the strength of a low signal and thus acts as an amplifier. Given Fig. Below shows the basic circuit of a BJT transistor amplifier.
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common emitter amplifier circuit

Bipolar Junction transistor as an amplifier

The low signal is applied in the input of Base-Emitter region and output is taken across the load collector resistance connected in the collector circuit. In order to achieve good amplification, the input Base-Emitter circuit should always remain forward biased. To do so, an external D.C voltage (VEE) is applied in the Base-Emitter circuit in addition to the signal as shown given below.

This D.C voltage VEE is known as the bias voltage and its magnitude is such that it always keeps the input B-E circuit forward biased. As the input B-E circuit has low resistance, therefore, a small change in input signal voltage causes a noticeable change in emitter current.

This causes the *same change in collector current (IE=IB+IC). And the collector current IC flowing through a load resistance produces a large voltage across it (Ohm’s Law). Thus, a low signal applied in the input B-E circuit appears in the amplified form in the output collector circuit. Hence in this way, a BJT transistor acts as an amplifier.

Practical Application of BJT Transistor as an Amplifier:

The action of a BJT (Bipolar Junction Transistor) as an amplifier can be made simpler. if we consider typical circuit values. Suppose collector load resistance RC = 5 kΩ. Let us further assume that a change of 0.1V in signal voltage produces a change of 2 mA in emitter current.

Obviously, the change in collector current would also be approximately 2 mA. This collector current flowing through collector load RC would produce a voltage = 5 kΩ × 2 mA = 10 V.

Thus, a change of 0.1 V in the signal has caused a change of 10 V in the output circuit. In other words, the transistor has been able to raise the voltage level of the signal from 0.1 V to 10 V i.e. voltage amplification factor (output/input)V is 100.
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 Bipolar Junction transistor as an amplifier

Working of BJT (Bipolar Junction Transistor) as a Switch:


It is the region in which BJT (Bipolar Junction Transistor) act as an ON switch, in this region both Base-Emitter and Base-Collector junctions of the BJT transistor are forward biased. In this region, very high currents flow through the output of transistor, as both input and output junctions of the BJT are forward biased and bulk resistance offered is very much less.

Hence BJT Transistor in the saturation region is considered as ON switch.


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When both junctions of BJT (Bipolar Junction Transistor) are made reverse bias then the transistor will act is an open switch, which means input base voltage will be zero IB=0, the transistor is in the cutoff region of its operation.

When both junctions are reverse bias there will be no base current.

Under this condition, there is a very small amount of collector current that is known as collector leakage current, ICEO, and this is due to thermally produced carriers. And it is extremely small and hence it is usually neglected in circuit analysis so that VCE= VCC.

In cutoff, neither the input Base-Emitter nor the output Base-Collector junctions are forward biased.

Note: A Bipolar Junction Transistor is mostly operated for amplification purpose, for Switching Purpose generally JFET and MOSFETs are used.

Advantages of BJT (Bipolar Junction Transistor) 

  • The BJT (bipolar junction transistor has a good response at higher frequencies.
  • The BJT transistor has a large amplification factor.
  • The BJT Transistor provides better voltage gain.
  • The BJT can be operated in three different regions (active, Saturation and cut-off region).

Disadvantages of BJT (Bipolar Junction Transistor)

  • They produce more noise.
  • BJTs are very sensitive to the heat.
  • The switching in Bipolar Junction Transistor is very slow as compared to FETs.

Applications of BJT (Bipolar Junction Transistor)

A Bipolar Junction Transistor has two main applications, one is Amplification and other is switching. They are mostly used as amplifiers in electronic circuits like Audio amplification, Voltage or current amplification. They are the heart of every electronic circuit because every electronic circuit requires audio, video, current or voltage amplification at some stage of their working so they are used there.

This is all about BJT (Bipolar Junction Transistor), In this article, we covered working of Bipolar Junction Transistor as an Amplifier and as a Switch.

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Also Read:

  1. Difference Between NPN and PNP Transistor
  2. Transistor Configuration, Common Emitter, Base and Common Collector
  3. Biasing of Transistor and Different Methods of Transistor Biasing
  4. Common Emitter Amplifier, Working and Applications
  5. Common Base Amplifier, Working and Applications
  6. Common Collector Amplifier, Working and Applications
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